a). Field of the Present Invention
The present invention relates to an image reproducing apparatus for recovering a video signal from codes compressed and coded in such a coding method, based on the discrete cosine transform (hereinafter, referred to as "DCT") as JPEG and MPEG.
b). Description of the Prior Art
Images are usually compressed and coded when digitally recorded on a CD-ROM or hard disk because the image digitization would otherwise yield a huge amount of data.
DCT-based coding methods are more frequently used among compressing and coding methods. Such a method has been adopted to international standard coding methods such as JPEG (Joint Photographic Expert Group) and MPEG (Moving Pictures Expert Group).
A conventional DCT-based image recovering from a bitstream coded by means of a DCT-based coding method, e.g. an MPEG bitstream, will be described with reference to a drawing. FIG. 1 is a diagram illustrating how images are recovered from image codes based upon MPEG codes. In FIG. 1, each of the codes are read to analyze the type of the code in a header analyzer 1. In MPEG codes, the codes are divided into three types--the intra-frame encoded codes which constitute an intra-coded picture as I picture, the inter-frame encoded codes which are predicted only from a preceding frame to constitute a predictive coded picture as P picture, and the inter-frame encoded codes which are predicted from preceding and succeeding frames to constitute a bi-directionally predictive coded picture as B picture.
If the read codes are of an I picture: the codes, that is, efficiently compressed variable-length Huffman codes are decoded in a decoder 2 and inverse-quantized in an inverse quantizer 3; the values of pixels in blocks into which the picture has been divided are calculated by inverse DCT processing in an inverse DCT (IDCT) stage 4; and the calculated values or expanded video data are written in a video memory 10 to display the picture.
If the read codes are of a P picture: the codes are decoded in a decoder 2, and inverse-quantized in an inverse quantizer 3; differences of the blocks are calculated by inverse DCT processing in an inverse DCT (IDCT) stage 4; each difference is added by a forward predictor 7 to a corresponding motion-compensated block of a preceding frame stored in a preceding frame stage 5; the resultant expanded video data is written in a video memory 10 to display the image.
Also, if the read codes are of a B picture: the codes are decoded in a decoder 2, and inverse-quantized in an inverse quantizer 3; differences of the blocks are calculated by inverse DCT processing in an inverse DCT (IDCT) stage 4; each difference is added by a bi-directional predictor 8 or backward predictor 9 to a corresponding motion-compensated block of a preceding frame stored in a preceding frame stage 5 and a motion-compensated block of a succeeding frame stored in a succeeding frame stage 6; the resultant expanded video data is written in a video memory 10 to display the image.
As described above, any video reproducing system according to the international standard MPEG code can recover the image from MPEG codes. However, a high speed video reproduction is impossible without a high speed CPU because inverse DCT processing and decoding charge the CPU with a heavy load. For example, if a JPEG or MPEG video reproducing system is to process 15 frames every second, the reproducing system has to complete a frame of recovery processing for about 66 ms. If the reproducing system takes 20 ms for decoding huffman codes, 10 ms for inverse quantization, 30 ms for inverse DCT, and 20 ms for display, this means that the entire processing takes 80 ms, resulting in a 14 ms delay in reproducing a frame of image.
In order to reduce the load of CPU, there can be devised a method in which the image is compressed in coding and expanded in decoding, or a method in which the calculation period in the inverse DCT is reduced by assuming high frequency (HF) components to be zero. However, such a method would eliminate HF components of the image, and accordingly cause the image to become dim, thereby increasing a degradation in the picture quality.
An exemplary apparatus for reducing a degradation in the picture quality is disclosed in Japanese Patent Publication No. Sho62-198269 (1987). In the disclosed apparatus, images which are akin in picture quality to the original images are reproduced by masking non-sharpness of HF component reduced codes at the time of expansion to emphasize HF components.
However, there is a problem in such an apparatus that taking too much time for non-sharpness masking processes prevents a high speed video reproduction.
Also, in Japanese Patent Publication No. Sho62-295583 (1987), the differences between an original image and the reproduced image are calculated in advance by using the same kind image signal, noise signals with a signal distribution according to the distribution of the differences are generated after expansion, and randomly added to the reproduced image to reconstruct the reproduced image.
However, there is also a problem in such a coding scheme as described above that the noise signals have to be generated from some kinds of images, which in turn makes the structure of video reproducing system complicated.
Further, in Japanese Patent Publication No. Hei1-213067 (1989), the image is reproduced in such a way that each block is divided by every frequency band into sub-blocks, which are independently vector-quantized, and expanded sequentially from the lower to the higher in frequency so as to make the image gradually clear.
However, the above coding scheme disadvantageously takes much time to make a reproduced image clear, and accordingly can not afford a high speed video reproduction.
In Japanese Patent Publication No. Hei3-283989 (1991), detecting with respect to each of the blocks of an already coded image the fineness of a pattern, the position, direction and sharpness of an edge, or the position and direction of occurrence of an orthogonal transform coefficient of which the absolute value is not less than a predetermined threshold when an orthogonal transform performs an image signal to each the blocks, the image is reproduced by changing in response to the detected contents the way of scanning orthogonal transform coefficients for decoding and quantization.
However, if the coding method is partially changed in the course of image data compression as described above, the resultant codes will not be compatible with an international standard coding method. Thus, such codes disadvantageously need an image reproducing apparatus exclusively used for the code.